Drag, terminal velocity and dark flight
After the bright fireball ends, a meteorite still has kilometres to fall. This lesson uses balanced and unbalanced forces to explain how fast it lands and where it ends up.
- Calculate a resultant force from weight and drag.
- Explain terminal velocity using balanced forces.
- Explain why big and small fragments land in different places.
Links to: KS3 physics, forces: balanced and unbalanced forces, resultant force, drag and its effect on motion.
Two forces
Once a meteorite has slowed down, two main forces act on it. Its weight pulls it down. Drag (air resistance) pushes upwards against its motion, and gets bigger the faster it falls.
If weight is bigger than drag, the resultant force is downwards and the meteorite speeds up. If drag is bigger, the resultant force is upwards and it slows down. If the two are equal, the forces are balanced, the resultant is zero and the meteorite falls at a constant speed called its terminal velocity.
Dark flight
At heights of roughly 30 to 10 km the meteorite has slowed so much that it stops glowing. From here to the ground it falls in dark flight at its terminal velocity. For stones of about 100 g to 10 kg this is roughly 50 to 150 m/s, which is fast enough to make a dent or punch through a roof, but nothing like the speed in space.
During dark flight the wind blows the meteorite sideways, so scientists must use weather data to predict where pieces land.
Strewn fields
Meteoroids often break apart in the air. The fragments all start dark flight from about the same place, but a large piece has more weight compared with its drag than a small piece, so it keeps more of its speed and travels further. Small pieces stop short.
The result is an oval area on the ground called a strewn field, with the smallest pieces at one end and the largest at the far end. Search teams use this pattern to decide where to look.
Is it still speeding up?
A 2.0 kg meteorite is falling in dark flight. At one moment the drag on it is 14 N. Take g = 10 N/kg. What is the resultant force, and what happens next?
- Weight: W = m × g = 2.0 × 10 = 20 N downwards.
- Resultant force = 20 N down − 14 N up = 6 N downwards.
- The force is unbalanced and downwards, so the meteorite speeds up. As it speeds up, drag increases.
- When drag reaches 20 N, the forces balance and it falls at terminal velocity.
Answer: A resultant of 6 N downwards. It accelerates until drag rises to 20 N, then falls at a constant terminal velocity.
Investigate terminal velocity with paper cake cases. Drop 1, 2, 3, 4 and 5 nested cases from 2 m and time each fall three times. Calculate mean times and speeds. Which variable did you change, which did you measure, and which did you keep the same? Explain your results using weight and drag.
Quiz
5 questions. Choose an answer to see whether you are right and why. Finish the quiz to complete the lesson.
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Question 1What is terminal velocity?
When drag equals weight, the resultant force is zero and the speed stops changing.
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Question 2A 3 kg meteorite (g = 10 N/kg) has 30 N of drag acting on it. What is it doing?
Its weight is 3 × 10 = 30 N, equal to the drag, so the forces are balanced.
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Question 3What is dark flight?
Once slowed to terminal velocity, the meteorite no longer glows and falls unseen.
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Question 4In a strewn field, where are the largest fragments usually found?
Large fragments have more weight compared with their drag, so they keep their speed and travel further.
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Question 5Why must scientists know the wind to predict where meteorites land?
The fall from about 20 km takes minutes, and winds at different heights carry the stone sideways.